If you have paid money for StaffPad and its in-app purchases, only Apple can take that away. So even if the new owners decide to discontinue the product (highly unlikely), you still have your Apple purchases. You might not get any future updates that make the app continue to work with the latest iOS. If this happened, I would just never again update my iPad. (I have no idea how things work on the Microsoft side.)
But there is no reason that the new owners would suddenly decide to stop making money off of the existing software. In no reasonable world would the current state of StaffPad be in jeopardy. And for my part, if it never received another update, I would continue to use it.
What is in question is how much the future vision of StaffPad will continue to be dictated by DWH. It is disconcerting that the sale of the app was not announced and a statement issued reassuring users of the future of the product. They had to know this would come out, and yet they chose to remain silent. This is, at best, a poor business decision. At worst, it is a nefarious plot by DAW manufactures to undermine the future of composers who actually read music.